This invention relates to machining a carbon-carbon structure.
In recent years, solid rocket motor assemblies have undergone a material transition. Initially, for example, common structural materials, such as steel, aluminum, and fiberglass were used to fabricate exit cones, with insulation to protect these materials from the high temperatures of the rocket motor exhaust. More recently, carbon-carbon exit cones have been developed. These structures can carry the operaring pressures and thermal loads of solid rocket motors while operating at temperatures in the range of 4000.degree. to 5000.degree. F. (2200.degree. to 2750.degree. C.).
Several methods are available for fabricating carbon-carbon exit cones. According to the method outlined by O'Driscoll et al, U.S. Pat. No. 4,477,024, a conventional (2D) woven carbon fiber fabric is impregnated with a suitable resin, such as a phenolic resin, and the resin-impregnated fabric is assembled in suitable fashion upon a male mandrel. The uncured laminate is transferred to a female die for curing and the male mandrel is withdrawn. Following curing of the resin, the now freestanding preform can be skin machined and prepared for carbonization.
According to another procedure, described by Inman et al, U.S. Pat. No. 4,519,290, carbon fiber strands are braided over a male mandrel. The braided preform is rigidized on the mandrel and prepared for carbonization.
The fiber preform is impregnated with pitch in a vacuum, and then subjected to heat and pressure to produce carbonization. This process is repeated until the structure has the desired density. The billet that is thus formed is machined to achieve the desired dimensions and finish of the final product.
The carbon-carbon exit cones present certain problems in manufacturing. In order to attach these cones to the rest of the motor structure, i.e., to the throat portion of the rocket motor, a threaded cylindrical section is required. I have found that in machining this threaded section, small portions of fiber chip out of the thread surface, particularly in the crest of the thread, leaving surface defects. Lest these thread defects lead to operational failure of the exit cone, I have discovered a method for machining non-defective threads in the attachment portion of a carbon-carbon rocket motor exit cone. More broadly, my discovery is applicable to the machining of threads in a carbon-carbon billet.
Accordingly, it is an object of the present invention to provide a method for machining threads in a carbon-carbon billet.
Other objects and advantages of the present invention will be apparent to those skilled in the art.